This invention relates to an optically readable information disc (or simply optical disc) for storing, recording and reproducing voices, images, information, and the like, characterized in its base plate.
A base plate for optical discs used in digital audio discs, video discs, optical disc recording media, magneto-optical disc recording media comprises a transparent plate of about 1 mm thick having an information pattern of groove-like and hole-like depressions and protuberances on a surface thereof.
Such an optical disc base plate can be formed by the following three methods:
(1) In a mold installed with a metal-made stamper having an information pattern such as grooves and pits (or holes), a melt of polymer material such as polycarbonate, polymethyl methacrylate, or the like, is injected, cooled and molded to give a transparent plate having an information pattern [Nikkei Mechanical, page 34, Feb. 1, 1982; Nikkei Electronics, page 133, June 7, 1982].
(2) A thin liquid layer of light-curable resin is interposed between a metal-made stamper having an information pattern and a transparent supporting plate made from plastics or glass, and exposed to light from the transparent supporting plate side to cure the resin, followed by separation of the cured resin from the stamper while maintaining the adhesion between the transparent supporting plate and the cured resin to give a transparent plate having an information pattern [Japanese Patent Unexamined Publication Nos. 53-86756 and 55-152028].
(3) After pouring a light-curable resin into a space formed by placing a stamper having an information pattern and a light-transmissible flat plate in parallel, light is irradiated from the light-transmissible flat plate side to cure the resin, followed by removal of the stamper and the light-transmissible flat plate to give a transparent plate having an information pattern [Japanese Patent Unexamined Publication No. 55-160338].
But these methods have the following problems.
The method (1) is good in workability, but it is difficult to completely remove molecular orientation at the time of flowing and solidification of the polymer material. Thus, optical anisotropy is generated in the base plate, resulting in causing noises at the time of reading or writing information signals. The optical anisotropy in the base plate becomes particularly remarkable when a heat distortion temperature of the base plate is raised in order to obtain a heat resistant base plate, which results in making optically practical use impossible. Further, the base plate obtained by the method (1) has a tendency to make it difficult to precisely transfer the shape of information pattern from the stamper to the base plate.
The method (2) is good in transfer properties of the information pattern, but is complicated in working steps and has a tendency to make the production cost higher since the transparent supporting plate should be produced previously. Further since it is difficult to maintain sufficient adhesiveness between the transparent supporting plate and the light-curable resin, there is a tendency to make it difficult to maintain reliability for a long period of time under high temperatures and high humidity.
The method (3) is good in workability, but is difficult to select light-curable resins and molding conditions. Thus, it is difficult to produce the base plate having excellent mechanical strength, heat resistance and moisture resistance and small optical anisotropy.